[This is my answer - slightly edited - to a question asked in a thread last week: "Engine oil article". Since the article addressed API service categories and didn't go into base stock oils in any detail, I think the subject deserves it's own thread]
The first thing you must understand is that the legal definition of "synthetic" is very broad, and the technical definition is vague. It has become mostly a marketing term and cannot be relied on to imply specifics about any product. Virtually any automotive fluid that is not derived from crude oil using only traditional refining processes going back to the 1920s can be called "synthetic". This includes fluids such as ethylene glycol based antifreeze and any brake fluid - both conventional glycol ether based or silicone.
Synthetic engine oil base stocks have two main advantages over traditionally refined engine oil base stocks:
1. Wider viscosity range without the use of viscosity index (VI) improvers, which can degrade with use, narrowing the viscosity range.
2. Greater oxidation resistance, which allows higher mileage between changes.
It is NOT "more slippery" than conventional oil.
It does NOT "cling" better to engine surfaces.
The above are two common internet myths.
The API currently segregates engine oil base stocks into five groups based on the type of refining process/oxidation resistance as defined in the API 1509 document and appendices that you can download from the API web site.
Group I: "Conventional" base stocks derived from crude oil distillation and solvent extraction processes that go back to the earliest days of crude oil refining.
Group II: Adds "hydroprocessing" , developed in the 1970s, to break up ring molecules and remove undesirable elements such as sulfur yielding more long chain saturated hydrocarbons, which means molecules consisting of only carbon and hydrogen atoms with only single bonds - no double bonds.
Group III: Additional hydroprocessing to remove additional undesirable elements as above resulting in a higher percentage of "pure" single bond chain hydrocarbons.
Group IV: Uses a complex petrochemical process to produce polyalpha olefins (PAOs).
Group V: Uses a complex petrochemical process to produce esters.
As one goes up in group number, the resulting base stock has wider viscosity index and more resistance to oxidation.
Group IV and V were developed for jet/gas turbine engines, which have no internal cooling and expose the lubricating oil to more heat resulting in higher oil temperatures, so lubricating oil requires very high oxidation resistance.
Group IV and/or V was the base used for the original Mobil 1 in the seventies.
In the eighties (or maybe it was the early nineties), Castrol introduced a "synthetic" engine oil using less expensive Group II and III base stocks, which allowed them to underprice Mobil.
Mobil sued Castrol asking the court to order Castol to not call their oil "synthetic", but Mobil lost the case, which forced them to reformulate Mobil 1 using Group II and III base stocks in order to remain price competitive.
As API service specifications evolved, the oxidation resistance requirement has become more strick, so even "conventional" engine oils that consist of mostly Group I base stocks have some Group II and III base stocks added to meet the current oxidation requirements. This applies to both S and C-category oils.
Base oils blended from Group II and III base stocks, which are both considered "synthetic" and "oil service life monitors" are what allow modern cars (like modern Corvettes) to accumulate 10-15K miles of daily use before the oil needs changing, however, for warranty coverage, GM still requires at least an annual change, regardless of mileage.
I generally recommend annual oil changes for vintage cars that accumulate a few hundred to a few thousand miles per year, but if the engine is tight, the PCV system effective, long enough trips to allow the oil to fully warm up, and the car is kept in a garage to protect it from condensing humidity, I don't see a problem going two years if only a few hundred miles are driven annually and the oil doesn't darken significantly in the first year.
Today's conventional base stock blends have much more oxidation resistance than when our cars were new, so the oil doesn't degrade as fast.
Also, since we typically only start and drive our vintage cars in mild weather we don't need a OW-40 engine oil that is suitable for every climate from arctic winters to tropical summers; 15W-40 is more than adequate for moderate to high ambient temperatures.
Based on the above facts, the question is: Is there a benefit to using "synthetic" oil given that it costs two to three times as much as conventional oil for either a freshly restored vintage engine or a high mileage original engine?
Now that everyone has the facts, each should be able to answer the question for themself.
Duke
The first thing you must understand is that the legal definition of "synthetic" is very broad, and the technical definition is vague. It has become mostly a marketing term and cannot be relied on to imply specifics about any product. Virtually any automotive fluid that is not derived from crude oil using only traditional refining processes going back to the 1920s can be called "synthetic". This includes fluids such as ethylene glycol based antifreeze and any brake fluid - both conventional glycol ether based or silicone.
Synthetic engine oil base stocks have two main advantages over traditionally refined engine oil base stocks:
1. Wider viscosity range without the use of viscosity index (VI) improvers, which can degrade with use, narrowing the viscosity range.
2. Greater oxidation resistance, which allows higher mileage between changes.
It is NOT "more slippery" than conventional oil.
It does NOT "cling" better to engine surfaces.
The above are two common internet myths.
The API currently segregates engine oil base stocks into five groups based on the type of refining process/oxidation resistance as defined in the API 1509 document and appendices that you can download from the API web site.
Group I: "Conventional" base stocks derived from crude oil distillation and solvent extraction processes that go back to the earliest days of crude oil refining.
Group II: Adds "hydroprocessing" , developed in the 1970s, to break up ring molecules and remove undesirable elements such as sulfur yielding more long chain saturated hydrocarbons, which means molecules consisting of only carbon and hydrogen atoms with only single bonds - no double bonds.
Group III: Additional hydroprocessing to remove additional undesirable elements as above resulting in a higher percentage of "pure" single bond chain hydrocarbons.
Group IV: Uses a complex petrochemical process to produce polyalpha olefins (PAOs).
Group V: Uses a complex petrochemical process to produce esters.
As one goes up in group number, the resulting base stock has wider viscosity index and more resistance to oxidation.
Group IV and V were developed for jet/gas turbine engines, which have no internal cooling and expose the lubricating oil to more heat resulting in higher oil temperatures, so lubricating oil requires very high oxidation resistance.
Group IV and/or V was the base used for the original Mobil 1 in the seventies.
In the eighties (or maybe it was the early nineties), Castrol introduced a "synthetic" engine oil using less expensive Group II and III base stocks, which allowed them to underprice Mobil.
Mobil sued Castrol asking the court to order Castol to not call their oil "synthetic", but Mobil lost the case, which forced them to reformulate Mobil 1 using Group II and III base stocks in order to remain price competitive.
As API service specifications evolved, the oxidation resistance requirement has become more strick, so even "conventional" engine oils that consist of mostly Group I base stocks have some Group II and III base stocks added to meet the current oxidation requirements. This applies to both S and C-category oils.
Base oils blended from Group II and III base stocks, which are both considered "synthetic" and "oil service life monitors" are what allow modern cars (like modern Corvettes) to accumulate 10-15K miles of daily use before the oil needs changing, however, for warranty coverage, GM still requires at least an annual change, regardless of mileage.
I generally recommend annual oil changes for vintage cars that accumulate a few hundred to a few thousand miles per year, but if the engine is tight, the PCV system effective, long enough trips to allow the oil to fully warm up, and the car is kept in a garage to protect it from condensing humidity, I don't see a problem going two years if only a few hundred miles are driven annually and the oil doesn't darken significantly in the first year.
Today's conventional base stock blends have much more oxidation resistance than when our cars were new, so the oil doesn't degrade as fast.
Also, since we typically only start and drive our vintage cars in mild weather we don't need a OW-40 engine oil that is suitable for every climate from arctic winters to tropical summers; 15W-40 is more than adequate for moderate to high ambient temperatures.
Based on the above facts, the question is: Is there a benefit to using "synthetic" oil given that it costs two to three times as much as conventional oil for either a freshly restored vintage engine or a high mileage original engine?
Now that everyone has the facts, each should be able to answer the question for themself.
Duke
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